Gas sensor and method for measuring a gas component in a gas mixture

ABSTRACT

A gas sensor based on solid electrolyte is proposed for measuring a gas component in a gas mixture, having at least one sensitive region, which has a first means for producing a reaction gas from an additional gas component of the gas mixture. A second means is situated in the sensitive region of the gas sensor, using which the residual content of the reaction gas may be determined, after a reaction that takes place between the reaction gas and the gas component to be measured.

[0001] The present invention relates to a gas sensor and a method formeasuring a gas component in a gas mixture according to the definitionof the species in the independent claims.

BACKGROUND INFORMATION

[0002] In the course of progressive environmental legislation, thedemand is growing for sensors with the aid of which even the smallestquantities of pollutants may be reliably determined. In this context,above all, gas sensors play a great role which make possible thedetermination of gaseous pollutants in the ppm range, independent of thetemperature of the measuring gas. The measuring signals of the gassensor proportional to the quantity of pollutant are, in thisconnection, often so small that great measuring inaccuracy cannot beavoided. A possible way out of this dilemma is represented by anindirect determination of pollutants.

[0003] Thus, a gas sensor may be seen in EP 241 751 A2, using which, thecontent of ammonia, carbon monoxide, hydrocarbons, nitrogen oxides orsulfur dioxide may be monitored in gas mixtures, but not the oxygencontent. To be able to determine nitrogen oxides, among other things, ameasuring method is proposed in which a known quantity of ammonia isadded to a gas mixture as reaction gas which reacts with the nitrogenoxides at a catalyst of the gas sensor. If the quantity of ammoniaoriginally added is known, one may conclude what the NOx concentrationin the gas mixture is, by determining the remaining content of ammonia.The disadvantage of this method is that a device for adding the ammoniahas to be provided.

[0004] It is the object of the present invention to make available a gassensor which makes possible measuring various gas components in a gasmixture, reliably and using accurate timing.

SUMMARY OF THE INVENTION

[0005] The gas sensor according to the present invention and the methodaccording to the present invention, having the characterizing featuresof the independent claims, advantageously attains the object on whichthe present invention is based. The determination of the gas componentto be measured is made indirectly via the determination of the remainingcontents of a reaction gas, after the latter has fully reacted with thegas component to be measured. It is of particular advantage that thereaction gas does not first have to be added to the gas mixture, whichwould require one or possibly more appropriate devices, but that it isgenerated in the gas sensor itself.

[0006] For this purpose, the gas sensor has a first means by which, in afirst step, a reaction gas is produced from another gas component of thegas mixture which is not the gas component to be measured. This reacts,in a second step, with the gas component to be measured. The gas sensoralso includes a sensitive region, in which a second means is situatedwhich, in a third step, permits the determination of the residualcontent of reaction gas after its reaction with the gas component to bemeasured. If the reaction gas is generated in excess as related to thegas component to be measured, and if the quantity of generated reactiongas is known, one may conclude from the residual content of reaction gaswhat the quantity of gas component to be measured originally was in thegas mixture.

[0007] It is advantageous if an electrochemical pumping cell is providedas the first means, at whose electrode facing the gas mixture thefurther gas component may be reduced or oxidized as needed while forminga reaction gas. As the second means, depending on the particularapplication, an electrochemical pump cell, an electrochemicalconcentration cell or a resistive measuring element come intoconsideration.

[0008] Furthermore, it is advantageous if a catalyst is provided in thegas sensor, which catalyzes the reaction of the gas component to bemeasured and the reaction gas. The catalyst may cover one of theelectrodes of the first or second means, at least to a great extent.

[0009] In a further advantageous specific embodiment of the presentinvention, an electrochemical pump cell is connected upstream of thefirst and second means which has the effect of regulating the oxygenproportion in the gas mixture before it reaches the sensitive region ofthe gas sensor. If the electrochemical pump cell is combined with anelectrochemical concentration cell, this increases the accuracy withwhich the oxygen proportion of the gas mixture may be regulated, and atthe same time makes possible the additional determination of the oxygenproportion in the gas mixture. The regulation of the oxygen proportiontakes place, for example, in a first region of a measuring gas chamberof the gas sensor, and the reaction of the gas component to be measuredwith the reaction gas as well as the determination of the residualcontent of reaction gas takes place in a second region of the measuringgas chamber.

[0010] The determination of the gas component to be measuredadvantageously takes place in such a way that first a reaction gas isgenerated from another gas component of the gas mixture, and the gascomponent to be measured is made to react with the reaction gas withinthe gas sensor. In this context, one should take care that the reactiongas is always present in excess with respect to the quantity of the gascomponent to be measured. After the reaction, the residual content ofthe reaction gas is determined, and from the residual content one mayconclude what the original concentration of the component to be measuredwas, knowing the quantity of reaction gas just generated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Nine exemplary embodiments of the present invention are shown inthe drawings and are explained in detail in the following description.The figures show:

[0012]FIG. 1: a cross section through the large surface of a sensorelement according to a first exemplary embodiment,

[0013]FIG. 2: a cross section through the large surface of a sensorelement according to a first variant of the first exemplary embodiment,in which the positions of first and second means and catalyst areinterchanged,

[0014]FIG. 3: a cross section through the large surface of a sensorelement according to a second variant of the first exemplary embodiment,the second means and the catalyst being situated in a separate layerplane of the gas sensor,

[0015]FIG. 4: a cross section through the large surface of a sensorelement according to a second variant of the first exemplary embodiment,the second means and the catalyst being situated in a separate layerplane of the gas sensor,

[0016]FIG. 5: a cross section through the large surface of a sensorelement according to a second exemplary embodiment, in which thecatalyst is integrated into the second means,

[0017]FIG. 6: a cross section through the large surface of a sensorelement according to a third exemplary embodiment, whose measuring gaschamber is subdivided by a diffusion barrier,

[0018]FIG. 7: a cross section through the large surface of a sensorelement according to a fourth exemplary embodiment, whose measuring gaschamber is subdivided by a diffusion barrier and in which the catalystis integrated into the second means,

[0019]FIG. 8: a cross section through the large surface of a sensorelement according to a fifth exemplary embodiment, in which thediffusion barrier for subdividing the measuring gas chamber is situatedbetween the first and second meams,

[0020]FIG. 9: a cross section through the large surface of a sensorelement according to a first variant of the fifth exemplary embodiment,the second means and the catalyst being situated in a separate layerplane of the gas sensor,

[0021]FIG. 10: a cross section through the large surface of a sensorelement according to a sixth exemplary embodiment, in which thedetermination of the gas component to be measured is made in apotentiometric manner,

[0022]FIG. 11: a cross section through the large surface of a sensorelement according to a seventh exemplary embodiment, in which thedetermination of the gas component to be measured is made in a resistivemanner,

[0023]FIG. 12: a cross section through the large surface of a sensorelement according to an eighth exemplary embodiment, in which thecatalyst is combined with the first means,

[0024]FIG. 13: a cross section through the large surface of a sensorelement according to a ninth exemplary embodiment, in which the catalystis combined with the first and the second means.

[0025] In FIGS. 1 through 13, the reference numerals used, if nototherwise noted, refer to structural components of a sensor elementhaving the same function.

[0026] Exemplary Embodiments

[0027] FIGS. 1 shows a basic design of a first specific embodimentaccording to the present invention. Designated by reference numeral 10is a planar sensor element of an electrochemical gas sensor, which, forexample, has a plurality of solid electrolyte layers 11 a, 11 b, 11 c,11 d, 11 e, 11 f and 11 g that conduct oxygen ions. In this context,solid electrolyte layers 11 a-11 g are designed as ceramic foils andform a planar ceramic body. The integrated form of the planar ceramicbody of sensor element 10 is produced in a manner known per se, bylaminating together the ceramic foils printed over with functionallayers and subsequently sintering the laminated structure. Each of thesolid electrolyte layers 11 a through 11 g is formed from solidelectrolyte material that conducts oxygen ions, such as ZrO₂ stabilizedpartially or fully with Y₂O₃. Solid electrolyte layers 11 a-11 galternatively may be replaced, at least partially, by foils made ofaluminum oxide, at places at which ion conduction in the solidelectrolyte is not important or even undesired.

[0028] Sensor element 10 includes a measuring gas chamber 13, which, viaa gas intake port 15, is in contact with a gas mixture surrounding thegas sensor. Gas intake port 15 is designed, for example, as a borepenetrating solid electrolyte layer 11 a, but it may also be situated inthe same layer plane 11 b as measuring gas chamber 13. Between gasintake port 15 and measuring gas chamber 13, and in the direction ofdiffusion of the measuring gas, a buffer chamber 17 and a diffusionbarrier 19 made of porous ceramic material are provided. Buffer chamber17 is used to avoid signal spikes in the case of rapidly changing gasconcentrations in the gas mixture. In a further layer plane 11 d of thesensor element, a reference gas channel 30 is formed, which holds areference gas atmosphere. The reference gas atmosphere may be air, forexample. For this purpose, reference gas channel 30 has an opening, notshown, on the side of the sensor element facing away from the measuringgas, which ensures gas exchange with the surrounding air.

[0029] Also, embedded in the ceramic base of sensor element 10, betweentwo insulating layers 32, 33 is a resistance heater 35. The resistanceheater is used for heating up sensor element 10 to a necessary operatingtemperature.

[0030] In first measuring gas chamber 13, one or two first innerelectrodes 20 are situated. At the outer side of solid electrolyte layer11 a, which directly faces the gas mixture, there is an outer electrode22, which may be covered by a porous protective layer (not shown).Electrodes 20, 22 form a first electrochemical pump cell. The operatingprocedure as pump cell includes applying a voltage between electrodes20, 22 of the pump cell, which results in an ion transport betweenelectrodes 20, 22 all the way through solid electrolyte 11 a. The numberof the “pumped” ions is directly proportional to a pump current flowingbetween electrodes 20, 22 of the pump cell.

[0031] In measuring gas chamber 13, a second and a third inner electrode24, 26 are provided downstream from electrode 20 in the diffusiondirection of the measuring gas. The common outer electrode which goeswith them, which acts as reference electrode 28, is located in referencechannel 30. In this context, second inner electrode 24 together withreference electrode 28 forms a second electrochemical pump cell, andthird inner electrode 26 together with reference electrode 28 forms athird electrochemical pump cell. In addition, inner electrode 20 may beconnected together with reference electrode 28 to form anelectrochemical Nernst or concentration cell. By a Nernst cell or aconcentration cell is generally understood a two-electrode system inwhich the two electrodes 20, 28 are expoesd to different gasconcentrations, and the difference of the potentials present atelectrodes 20, 28 is measured.

[0032] According to the Nernst equation, this potential differencepermits making an inference on what the gas concentrations are atelectrodes 20, 28.

[0033] A further possibility is connecting second inner electrode 24 toouter electrode 22 to form a second electrochemical pump cell andconnecting third inner electrode 26 to outer electrode 22 to form athird electrochemical pump cell.

[0034] The electrode material for all electrodes is applied in agenerally known way as cermet in order to sinter the electrode materialto the ceramic foils.

[0035] In order to operate sensor element 10 as gas sensor, the firstpump cell is drawn upon, together with the concentration cell forregulating the oxygen proportion of the gas mixture that has diffusedinto measuring gas chamber 13. A constant partial pressure of oxygen of,for instance, 0.1 through 1000 ppm is set in measuring gas chamber 13 bypumping in or pumping out oxygen. Control of the partial pressure ofoxygen in measuring gas chamber 13 is carried out by the concentrationcell. In this context, the pump voltage at the pump cell is varied insuch a way that, between electrodes 20, 28 of the concentration cell, aconstant potential difference sets in. In this context, the pump currentflowing in the pump cell is a measure of the oxygen concentrationpresent in the gas mixture that is diffusing in, and makes possible theadditional function of the gas sensor as an oxygen probe. Sincepremature decomposition of the gas component to be measured isundesirable at first inner electrode 20, first inner electrode 20 ispreferably made of a catalytically inactive material, such as gold or agold-platinum alloy. If the application of the gas sensor is limited todetermining stable gas components, inner electrode 20 mentioned may alsohave in it platinum, a rhodium-platinum alloy or another suitablematerial.

[0036] If one makes the assumption that the gas mixture present has onlya low oxygen proportion, one can do without the first inner electrodeand consequently also the first electrochemical pump cell. This is thecase, for example, in exhaust gases of motor vehicles which areconstantly operated having a lambda value=1. The sensor construction isthereby made simpler.

[0037] The gas mixture in measuring gas chamber 13, which is set to aconstant oxygen partial pressure, now reaches sensitive region 40 of thegas sensor. In this region, second inner electrode 24 of the second pumpcell is situated. At second inner electrode 24, which preferably, butnot necessarily, also has a catalytically inactive material, such asgold or a gold-platinum alloy in it, by applying an appropriate voltage,a reaction gas is generated from an additional gas component of the gasmixture, which is not the gas component to be measured, and this isreacted with the gas component to be measured. If the gas sensor isbeing used, for example, for determining nitrogen oxides, a potentialsuch as −500 to −750 mV is set with respect to reference electrode 28 atsecond inner electrode 24, and water and carbon dioxide are reduced tohydrogen and carbon monoxide. The oxygen set free in this context isreduced electrochemically and pumped off.

H₂O+2e ⁻

H₂+O²⁻ _((pumped off)) (e ⁻=electron)   (1)

CO₂+2e⁻

CO+O²⁻ _((pumped off))   (2)

[0038] Second electrode 24 is dimensioned so that the generated reactiongas (hydrogen and carbon monoxide) is present in excess with respect tothe quantity of gas components (nitrogen oxides) to be measured,contained in the gas mixture. In order to avoid having the gas componentto be measured (nitrogen oxides) decomposed too, on account of thestrong negative potential of second inner electrode 24, and consequentlyno longer being available for being measured, second inner electrode 24is preferably provided with a protective device 36. As shown in FIG. 1,protective device 36 may be formed, for example from solid electrolytematerial or another suitable type of ceramic material. The geometricaldesign of protective device 36, in the form of a cover layer that isslotted or furnished with a hole, has the effect that only a small partof the gas mixture that has diffused in comes into contact with secondinner electrode 24. Since even this small part of the gas mixture has asufficiently high proportion of the additional gas components (water,carbon dioxide), nevertheless an excess of reaction gas can always bemade available. Gas mixtures which contain air, for example, or exhaustgases of internal combustion engines, satisfy this assumption.

[0039] The gas mixture enriched with the reaction gas (hydrogen andcarbon monoxide) now reaches a part of sensitive region 40 facing awayfrom gas intake port 15. There, in measuring gas chamber 13, a catalyst38 is applied in the form of a catalytically active layer, whichcatalyses the reaction of the reaction gas (hydrogen and carbonmonoxide) with the gas component to be measured (nitrogen oxides)according to equation (3), (4).

xH₂+NO_(x)

x H₂O+½N₂   (3)

xCO+NO_(x)

x CO₂+½N₂   (4)

[0040] Since the reaction gas is present in excess, the completereaction of the gas component to be measured is ensured. On the side ofsensitive region 40 facing away from gas intake port 15, there is alsosituated a third inner electrode 26, which forms the third pump celltogether with reference electrode 28. Third inner electrode 26 mayoptionally be mounted on an additional solid electrolyte layer 37, inorder to shorten the diffusion path between catalyst 38 and third innerelectrode 26.

[0041] The potential of inner electrode 26 is selected so that oxygenfrom reference gas channel 30 is pumped to third inner electrode 26 andreacts there with the remaining reaction gas. Since, in this reaction,the reverse reaction of reaction (1), (2) is involved, the additionalgas component (water and carbon dioxide) re-forms (equation (5), (6)).For this, a potential of −300 to −500 mV is set at third inner electrode26.

H₂+O²⁻

H₂O+2e⁻  (5)

CO+O²⁻

CO₂+2e⁻  (6)

[0042] Third inner electrode 26 is made of a catalytically activematerial, such as platinum or an alloy of platinum, rhodium and/orpalladium. The pump current flowing in the third pump cell isdetermined, and is directly proportional to the residual concentrationof the reaction gas. Since the initial concentration, originallyproduced at second inner electrode 24, of the reaction gas in the gasmixture is approximately constant, and may be simply determined by acalibrating measurement, one may draw a conclusion on the originalcontent of the gas component to be measured present in the gas mixture,from the difference of the initial concentration and the residualconcentration. The smaller the measured residual concentration of thereaction gas is, the greater was originally the concentration of the gascomponent to be measured present in the gas mixture.

[0043] The application of a gas sensor having sensor element 10 is notlimited to the determination of nitrogen oxides. Basically, reactiongases may be produced either by electrochemical reduction or oxidation,using the second pump cell. In the first case, reduceable gas componentsmay be determined, and in the second case oxidizable ones.

[0044] If a reducing potential is set at second inner electrode 24 ofthe second pump cell, not only hydrogen and carbon monoxide may beproduced as reaction gases, but in principle also nitrogen monoxide fromnitrogen dioxide, or sulfur monoxide from sulfur dioxide or trioxide.

NO_(x)+2x e⁻

NO+x O²⁻ _((pumped off))   (7)

SO_(x)+2x e⁻

SO+x O²⁻ _((pumped off))   (8)

[0045] The reaction gases produced according to equations (7), (8) maybe reacted with reduceable gas components, and consequently drawn uponfor their determination. The selection of the reaction gas suitable foran individual case depends on the electrochemical standard potentials ofthe redox reactions running during the production and reaction of thereaction gas, and also on reaction-kinetic criteria.

[0046] Measuring oxidizable gas components is also possible, without anychange being necessary in the specific embodiment of the gas sensor.Only the potential of second inner electrode 24 is now selected, so thatone or more gas components of the gas mixture may be selectivelyoxidized at suitable temperatures. These may be, for instance, water,nitrogen monoxide, sulfur monoxide or sulfur dioxide.

N₂+2O²⁻

2NO+4e⁻  (9)

NO+O²⁻

NO₂+2e⁻  (10)

SO+xO²⁻

SO_(x)+2xe ⁻ x=1, 2   (11)

2O²⁻

O₂+2e⁻  (12)

[0047] At catalyst 38 a reaction then takes place of the reaction gasacting in oxidizing fashion with the gas reducing components to bedetermined, such as ammonia, hydrogen, methane or hydrocarbons.

3O₂+CH₄

CO₂+2H₂O   (13)

3NO₂+4NH₃

6H₂O+3.5N₂   (14)

2NO₂+CH₄

CO₂+2H₂O+N₂   (15)

[0048] In order to be able to determine the residual content of theoxidizing acting reaction gas at third inner pump electrode 26, thepotential of this electrode with respect to reference electrode 28 isselected in such a way that the residual content of reaction gas atthird pump electrode 26 is reduced and the oxygen being liberated in theprocess is pumped off into reference gas channel 30. The pump currentnow appearing with an opposite sign is utilized as the measuring signal.From the difference of the initial concentration present at first andthe residual concentration of the reaction gas remaining after thereaction, one may come to a conclusion on what was the concentration ofgas component to be measured originally present in the measuring gas.Thus, depending on the selection of the potentials at inner electrodes24, 26, the present gas sensor is suitable for determining both reducingand oxidizing gas components of a gas mixture.

[0049] If a reducing potential is set at second inner electrode 24 andan oxidizing potential is set at third inner electrode 26, oxidizing gascomponents may be determined, using the gas sensor. If an oxidizingpotential is set at second inner electrode 24 and a reducing potentialis set at third inner electrode 26, reducing gas components may bedetermined. In this context, the second inner electrode has a selectiveeffect with regard to the production of oxygen, and prevents oxidationof the detectable gas components.

[0050] By fine tuning the oxidizing and reducing potential at secondinner electrode 24, selectively determined oxidizing or reducingreaction gases, or mixtures of various oxisizing or reducing reactiongases may be produced. The potentials to be set for this, taking intoconsideration possible overvoltages, come about from the standardpotentials of the reactions, in which the required reaction gases areformed from additional gas components.

[0051] Since the potentials present at electrodes 24, 26 may be variedquickly, there is also the possiblity of determining, periodically or atshort time intervals, one or more reducing or oxidizing gas components,alternatingly one after another, using one sensor.

[0052] In another specific embodiment it is provided that second innerelectrode 24 is contacted by a constant current source, a largeelectrical resistor, for example, in comparison to the electricalresistance of second inner electrode 24 being provided between currentsource and electrode 24, in such a way that the predominant part of theelectric voltage present at the electrical resistor and at electrode 24falls off at the electrical resistor.

[0053] Since the quantity of the reaction gas produced at second innerelectrode 24 is a function of the electrode current of electrode 24,applying a constant current makes it possible to make available of aconstant volume of reaction gas per unit of time. To make possible anoperation of the sensor element that is as effective as possible, it isuseful, on the one hand, to select the potential applied to second innerelectrode 24 so that the electrochemical production of the desiredreaction gas from an additional gas component is ensured, and, on theother hand, to limit the electrode current at electrode 24 in such a waythat the reaction gas produced is present in slight excess with respectto the gas component to be measured. In this way, the sensor signal ofthe sensor element becomes independent of the concentration ofadditional gas components from which the reaction gas is produced atsecond inner electrode 24.

[0054] In FIGS. 2, 3 and 4 variants of the sensor element shown in FIG.1 are illustrated. In the variant shown in FIG. 2, second inner pumpelectrode 24 as well as protective device 36 are shifted to the part ofsensitive region 40 of the sensor element that faces away from gasintake port 15. By contrast, catalyst 38 and third inner pump electrode26 are situated on the side of sensitive region 40 facing gas intakeport 15. Since, in this variant, the gas component to be measured insensitive region 40 immediately hits catalyst 38, without first passingsecond inner pump electrode 24, the probability is very low that the gascomponent to be measured in an undesired manner reaches second innerpump electrode 24 without being converted at catalyst 38. A sufficientavailability of the reaction gas is ensured in this variant too, sincethe additional gas component required for it is able to advance withouthindrance to second inner electrode 24.

[0055] A second variant of the sensor element according to the firstspecific embodiment is shown in FIG. 3. The sensor element includes twoadditional solid electrolyte layers 11 c 1, 11 c 2. In layer 11 c 1there is a further measuring gas chamber 14, which is in contact withfirst measuring gas chamber 13 via a cut through through solidelectrolyte layer 11 c. In second measuring gas chamber 14 there arecatalyst 38 and third pump electrode 26. This layout of the sensorelement effects an extending of the diffusion path within sensitiveregion 40 of the sensor element, without the sensor element having to belengthened at the same time. The longer diffusion path effects adecoupling of the production of the reactive gas from its reaction withthe gas component to be measured and from the detection of the residualcontent of the reaction gas. FIG. 4 shows a third variant of the sensorelement according to a first specific example. Its construction has thelayer sequence of the sensor element shown in FIG. 3. Second inner pumpelectrode 24 as well as protective device 36 in this variant are locatedin second measuring gas chamber 14, and catalyst 38 and third pumpelectrode 26 are situated in first measuring gas chamber 13. Thisvariant combines the advantages of the first variant according to FIG. 2with the advantages of the second variant according to FIG. 3. The factthat the gas component to be measured in sensitive region 40 immediatelyreaches catalyst 38, without first passing second inner pump electrode24, and the extended diffusion path between catalyst 38 and second innerelectrode 24 minimize the probability that the gas component to bemeasured is able to reach second inner electrode 24.

[0056]FIG. 5 shows a sensor element according to a second specificembodiment of the present invention. Instead of a catalyst 38 and aseparate third inner electrode 26, the sensor element has acatalytically active, preferably porous combination electrode 27. Thisis situated in the part of sensitive region 40 facing away from gasintake port 15. Combination electrode 27 may be designed in the form ofan electrode partially or completely covered by a catalytically activelayer, or may be made completely of a catalytically active, preferablyporous material. Combination electrode 27 has the advantage that thespatial separation of the reaction of the reaction gas with the gascomponent to be measured and the detection of the residual content ofreaction gas is abolished. In addition, with the aid of combinationelectrode 27, the reaction of reaction gases and gas components to bemeasured may even be catalyzed, which are not able to be catalyzed onlyusing a catalytically active material, but additionally require theapplication of a corresponding potential to the catalyst. Furthermore,itis advantageous that, when manufacturing a sensor element according tothe second exemplary embodiment, in comparison to the variants describedbefore, one working step, which includes the application of a separatecatalyst 38, is avoided.

[0057] One variant of the sensor element shown in FIG. 5 is, analogousto the variant shown in FIG. 2, to undertake an exchange of innerelectrode 24, which produces reaction gas, and protective device 36 withcombination electrode 27. Second inner pump electrode 24 and protectivedevice 36 is consequently positioned in the part of sensitive region 40of the sensor element that faces away from gas intake port 15. At thesame time, combination electrode 27 is provided in the part of sensitiveregion 40 facing gas intake port 15. In this variant, the probabilitythat the gas component to be measured comes into contact with secondinner pump electrode 24, and is lost for the measurement, is minimized.

[0058] In a further variant of the sensor element shown in FIG. 5, whichlargely corresponds to the one already shown in FIG. 3, combinationelectrode 27 is shifted to a separate layer 11 c 1. Here too, therecomes about an extension of the diffusion path between inner electrode24 producing reaction gas and combination electrode 27, while at thesame time maintaining the linear extension of the sensor element.

[0059] A third variant of the sensor element described in FIG. 5,analogously to the third variant of the first specific embodimentalready illustrated in FIG. 4, includes the positioning of second innerpump electrode 24 and protective device 38 in separate solid electrolytelayer 11 c 1. This variant combines the advantages of the two variantsalready described of the sensor elements shown in FIG. 5 with oneanother.

[0060]FIG. 6 illustrates a sensor element according to a third exemplaryembodiment of the present invention, in which the measuring gas chamber13 additiuonally includes a diffusion barrier 42, which subdividesmeasuring gas chamber 13 into region 44, regulating the oxygen contentof the measuring gas and sensitive region 40. In the specificembodiments of the sensor element described so far, the possibilitybasically exists that reaction gas penetrates into the part of measuringgas chamber 13 facing gas intake port 15, in spite of protective device36. in this context, if a reducing reaction gas is involved, it isconverted because of the higher oxygen content in that location; if anoxidizing reaction gas is involved, it is decomposed at first innerelectrode 20. Diffusion barrier 42 impedes an undesired diffusion of thereaction gas produced at second inner electrode 24 into the part ofmeasuring gas chamber 13 facing gas intake port 15. This increases themeasuring accuracy of the sensor element, since the concentration of thereaction gas produced is dependent, in sensitive region 40 of the sensorelement only upon the quantity diffusing in of the gas component to bemeasured. One variant of the sensor element shown in FIG. 6 is toundertake an exchange of the positions of inner electrodes 24, 26 andprotective device 36 and catalyst 38, according to the sensor elementalready shown in FIG. 2. Similar to that in FIG. 3, a further variant isbased on the positioning of third inner electrode 26 and catalyst 38 ina separate layer plane 11 c 1. A third variant comes about, comparablyto the variant shown in FIG. 4, because of the positioning of secondinner electrode 24 and protective device 36 in second test electrolytelayer 11 c 1.

[0061]FIG. 7 shows a sensor element according to a fourth specificembodiment of the present invention. The sensor element shown in FIG. 7combines the advantages of features of the second with those of thethird exemplary embodiment. It includes both a diffusion barrier 42between oxygen regulating and sensitive region 40, 44 of measuring gaschamber 13 and the combination of catalyst 38 and third inner electrode26 to form combination electrode 27. Here too, an exchange of thepositions of second inner electrode 24 and protective device 36 andcombination electrode 27 is possible, as well as the shifting ofcombination electrode 27 or second inner electrode 24 into a separatelayer plane 11 c 1.

[0062]FIG. 8 a sensor element according to a fifth specific embodimentof the present invention may be seen. The sensor element shown in FIG. 8is based on that shown in FIG. 6, and inside measuring gas chamber 13 ithas a diffusion barrier 46 which is connected downstream from secondinner pump electrode 24, in the flow direction of the gas mixture, andsubdivides sensitive region 40 spatially into a part facing gas intakeport 15 and a part facing away from gas intake port 15. Such a specificembodiment is considered, above all, for application cases in which thereaction gas produced is inert to oxygen, or rather, is not subject toany decomposition at first inner electrode 20. On account of diffusionbarrier 46, diffusion to catalyst 38 and to third inner electrode 26 ismade more difficult, so that the effect of an extended dissusion pathbetween second and third inner electrodes 24, 26 is made even moredifficult.

[0063] If in addition, as shown in FIG. 9, catalyst 38 and third innerpump electrode 26 are moved into a separate layer plane 11 c 1,according to this first variant of the fifth exemplary embodiment, theeffect of a diffusion made more difficult is further reinforced by theapplication of diffusion barrier 46 in breakthrough 16 of solidelectrolyte layer 11 c. The diffusion made more difficult leads to theelimination of inhomogeneities within the gas mixture.

[0064] A further variant is to combine catalyst 38 and third innerelectrode 26 to form a combination electrode 27, which, for example, maybe shifted into separate layer plane 11 c 1, just as was second innerelectrode 24.

[0065] Variations in the sensor element on which the present inventionis based, while maintaining the measuring principle, are also thesubject matter of the present invention. Thus, measuring gas chamber 13,14 may optionally be filled with porous material as the diffusionresistor, or it may include a plurality of diffusion barriers. Inaddition, more than one electrochemical cell for regulating the oxygencontent of the measuring gas may be provided, or more than oneelectrochemical cell for producing a reducing or oxidizing reaction gasmay be provided, respectively. The means used for determining theresidual content of the reaction gas may also be provided in a pluralityof designs.

[0066] The detection of the residual content of reaction gas in the gasmixture is performed amperometrically, in the case of the exemplaryembodiments described, using the third pump cell. However, it ispossible to make the detection potentiometrically, using a concentrationcell. To do this, third inner electrode 26 may be connected togetherwith reference electrode 28 to form a Nernst cell or a concentrationcell.

[0067] The potentiometric detection of reaction gases, such as hydrogenor carbon monoxide is made particularly advantageously by using aso-called disequilibrium sensor. Such a sensor element is represented inFIG. 10. In measuring gas chamber 13 there is additionally a fourthinner electrode 29, which is catalytically inactive, and which isconnected together with catalytically active third inner electrode 26 toform a Nernst cell or a concentration cell. Since at catalyticallyactive third inner electrode 26 a different potential develops than atcatalytically inactive fourth inner electrode 29, a voltage may beascertained as measuring signal. This effect is especially pronounced ifa combination electrode 27 is used as the third inner electrode, whilecatalyst 38 is omitted.

[0068] A further possibility for detecting the reaction gas is by usinga resistive measuring element. A corresponding exemplary embodiment isrepresented in FIG. 11. A voltage is applied to third and fourthelectrodes 26, 29, and the resistance of a gas-sensitive layer 50 isdetermined between the two inner electrodes 26, 29.

[0069] In an additional advantageous embodiment, according to FIG. 12,catalyst 38 is combined with second inner electrode 24. In this context,the catalyst may cover second inner electrode 24 partially orcompletely, a porous solid electrolyte layer 48 being preferablypositioned between catalyst 38 and the surface of second inner electrode24, so as to prevent the gas component to be measured from beingelectrochemically converted at catalyst 38. The combination of catalyst38 and second inner electrode 24 especially effectively prevents theaccess of the gas component to be measured to second inner electrode 24,since the gas component to be measured first has to pass porous catalyst38 before it reaches second inner electrode 24. In catalyst 38 it meetsan excess of reaction gas diffusing in the opposite direction, and iscompletely converted.

[0070]FIG. 13 shows a sensor element according to a ninth exemplaryembodiment. In this context, catalyst 38 is combined both with secondinner electrode 24 and with third inner electrode 26. This setup leadsto a particularly effective avoidance of the diffusion of the gascomponent to be measured to second inner electrode 24.

[0071] In order to ensure an especially favorable gas flow within thegas sensor, in addition to protective device 36, one or more devices maybe provided for steering the gas flow within measuring gas chamber 13,14.

[0072] Application possibilities of the gas sensor on which the presentinvention is based are to be seen, for example, in pollutant detectionin the exhaust gases of internal combustion engines. In this regard, thedetection of nitrogen oxides particularly makes possible control of, forinstance, the service condition or the load state of an NOx storagecatalyst. To do this, the gas sensor is mounted in an exhaust systembranch, downstream in the flow direction from the NOx storage catalyst.In addition, the control of SCR systems operated using ammonia or ureais made possible by the determination of ammonia. In this context, thegas sensor is positioned in the exhaust system branch between theexhaust gas aftertreatment unit and the exhaust, and the ammonia contentof the issuing exhaust gas is controlled.

[0073] The gas sensor may also be used for pollutant analysis incombustion systems used for heating purposes. In addition, there is thepossibility of testing for the completeness of combustion by thedetection, for instance, of methane.

[0074] Basically, the gas sensor makes possible both the purelyqualitative detection of the existence of a gas component to be measuredand the determination of its concentration in a gas mixture.

What is claimed is:
 1. A gas sensor based on solid electrolyte formeasuring a gas component in a gas mixture, having at least onesensitive region, wherein the gas sensor has a first means forgenerating a reaction gas from another gas component of the gas mixture;and a second means is situated in the sensitive region (40), using whichthe residual content of the reaction gas may be determined, after areaction that takes place between the reaction gas and the gas componentto be measured.
 2. The gas sensor as recited in claim 1, wherein thefirst means is an electrochemical pump cell which has an electrode (24)facing the gas mixture.
 3. The gas sensor as recited in claim 2, whereina constant current is applied to the electrode (24) which faces the gasmixture.
 4. The gas sensor as recited in claim 2, wherein, at theelectrode (24) of the electrochemical pump cell, which faces the gasmixture, the additional gas component water is able to be reduced to thereaction gas hydrogen, and/or the additional gas component carbondioxide is able to be reduced to the reaction gas carbon monoxide. 5.The gas sensor as recited in claim 2, wherein, at the electrode (24) ofthe electrochemical pump cell, which faces the gas mixture, as anadditional gas component nitrogen monoxide and/or a sulfur oxide areeach able to be oxidized to a reaction gas.
 6. The gas sensor as recitedin one of claims 1 through 5, wherein the second means is anelectrochemical pump cell.
 7. The gas sensor as recited in one of claims1 through 5, wherein the second means is an electrochemicalconcentration cell.
 8. The gas sensor as recited in one of claims 1through 5, wherein the second means is a resistive measuring element. 9.The gas sensor as recited in one of the preceding claims, wherein acatalyst (38) is provided for the reaction of the reaxtion gas with thegas component to be measured.
 10. The gas sensor as recited in claim 9,wherein the catalyst (38) is situated in direct proximity to the secondmeans.
 11. The gas sensor as recited in claim 9, wherein the catalyst(38) to a great extent covers an electrode (24, 26, 27, 29) of the firstand/or of the second means.
 12. The gas sensor as recited in one of thepreceding claims, wherein an electrochemical pump cell for regulatingthe oxygen proportion in the gas mixture is preconnected to the firstand/or the second means.
 13. The gas sensor as recited in claim 12,wherein the electrodes (20, 22) of the electrochemical pump cell forregulating the oxygen proportion are situated in a different layer plane(11 a, 11 b) of the gas sensor from the catalyst (38) and/or theelectrodes (24, 26, 27, 28, 29) of the first and/or the second means.14. The gas sensor as recited in one of the preceding claims, wherein ameasuring gas chamber (13, 14) surrounded by solid electrolyte layers(11 a, 11 b, 11 c, 11 c 1, 11 c 2) is provided, to which the gas mixturemay be supplied via a diffusion resistor (19, 42), and in which theelectrodes (20, 24, 26, 27, 29) of the first means, of the second meansor of the electrochemical pump cell for regulating the oxygen proportionand/or the catalyst (38) are located.
 15. The gas sensor as recited inclaim 14, wherein the measuring gas chamber (13) is subdivided into tworegions (40, 44) and includes a diffusion barrier (42) as thesubdivision.
 16. The gas sensor as recited in one of the precedingclaims, wherein a means (36) is provided for steering the diffusing gasmixture.
 17. A method for measuring a component of a gas mixture,especially using a gas sensor according to one of claims 1 through 15,wherein an excess of a reaction gas is produced from an additionalcomponent of the gas mixture which is made to react with the componentto be measured; the residual content of the reaction gas is determinedafter the reaction; and from the residual content of the reaction gas,the original concentration of the component of the gas mixture that isto be measured is concluded.
 18. The method as recited in claim 17,wherein, the reaction gas is produced from the additional component ofthe gas mixture by reduction or oxidation.
 19. The method as recited inclaim 17 or 18, wherein the reaction gas is produced from the additionalcomponent of the gas mixture in an inner region (13, 14) of the gassensor which is separated to a great extent from a gas chambersurrounding the gas sensor.
 20. The method as recited in one of claims17 through 19, wherein a first reaction gas is produced by reduction fordetermining a first gas component to be measured, and alternating withthis, a second reaction gas is produced by oxidation for determining asecond gas component to be measured.
 21. The method as recited in one ofclaims 17 through 20, wherein the reaction gas is produced instoichiometric or volumetric excess with respect the quantity of gascomponent to be measured.
 22. The method as recited in one of claims 17through 21, wherein in addition the oxygen content of the gas mixture isdetermined.
 23. The use of the gas sensor according to one of claims 1through 16 and/or of a method according to one of claims 17 through 22for determining the concentration of a gas component in a gas mixture.24. The use of the gas sensor according to one of claims 1 through 16and/or of a method according to one of claims 17 through 22 fordetermining a gas component in the exhaust gas of an internal combustionengine.
 25. The use of the gas sensor according to one of claims 1through 16 and/or of a method according to one of claims 17 through 22for determining nitrogen oxides and/or ammonia.
 26. The use of the gassensor according to one of claims 1 through 16 and/or of a methodaccording to one of claims 17 through 22 for monitoring the servicecondition and/or the load state of an NOx storage catalyst.